Cost minimization of deoxygenation for control of Vampirovibrio chlorellavorus in Chlorella sorokiniana cultures
Deoxygenation is a strategy for preventing Vampirovibrio chlorellavorus infection of Chlorella sorokiniana cultures. Deoxygenation cost was minimized by sparging with nitrogen gas for only one hour at the onset of night and relying on naturally-occurring dark-respiration to maintain low oxygen concentrations throughout the night. This technique substantially reduced V. chlorellavorus infection in laboratory co-cultures. The cost of the approach was evaluated in outdoor experiments with one pure water and one pathogen-free C. sorokiniana culture, grown in a 150-L sealed, vented, translucent-polyethylene reactor. Unlike the small laboratory reactors, which were also sealed and vented, the much larger outdoor reactor maintained the dissolved oxygen concentration in the C. sorokiniana culture at near zero concentration throughout the night cycle. This demonstrates that large covered tanks in commercial applications would keep oxygen concentration near zero for the entire night. The total nitrogen sparged, per night, per liter of algal culture, was determined for the 150-L reactor, and used to estimate the cost per m2 for large scale raceways. Onsite nitrogen generators are the most cost-effective method to supply a high flow of nitrogen gas to commercial scale raceways. The cost of deoxygenation treatment ranged from $16/ton AFDW algae for a shallow (2 cm) gravity flow system with 75% harvest prior to deoxygenation to over $1,300/ton for a 20 cm depth raceway with plastic tanks.